Various means for mixing solutions are known in the art. Both intrusive and non-intrusive means have been used to mix solutions, including colloidal suspensions, to prevent separation of homogeneous solutions into constituent components and/or to reconstitute solutions that have separated into constituent elements. Intrusive mixing devices, or those objects and devices which are inserted into a solution to agitate the solution with the assistance of an external power source, are well known. Such devices involve the use of intrusive mechanical mixers powered by electric or pneumatic motors. These devices provide relatively high torque and/or rotation of the solution and may result in adverse effects on the solution as a result of the formation of a significant vortex or whirlpool in the solution.
In some chemical environments, further adverse effects of intrusive agitation can be seen in the form of foaming or gelling of the body of solution while it is being mixed in a mixing tank or similar holding vessel. Such foaming or gelling may change the parameters of solutions' various chemical compositions and adversely affect their performance. Additionally, intrusive mixing devices and methods may introduce air into the mixture or solution and may cause oxidation of certain chemicals mixtures thereby changing the chemical reactivity of the solution.
Moreover, intrusive mixing devices frequently are comprised of metallic alloys, which may interact unfavorably with solutions of various chemical compositions. Over time, residual wear from a shaft used in an intrusive mixing system may cause the introduction of impurities into solutions as they are mixed. Coated shafts with chemically compatible plastic material last for longer periods of time, but still are problematic when used in abrasive solutions, such as slurry used in chemical mechanical polishing (CMP) of semiconductor wafers, because the abrasive characteristics of such solutions may wear on the coatings and, again, cause introduction of impurities into the solutions during mixing.
Moving parts in intrusive mixing devices (such as bearings, pins, and contact surfaces) also require periodic maintenance. Pneumatic motors demand continual lubrication by an in-line lubricator device. Additionally, exhaust from such motors must be vented to prevent atmospheric contamination when used in a clean room environment; the need to engage in such venting may contribute to an increase in routine maintenance. Finally, the space required for housing such motors and power supplies and the size of the motors and power supplies may create safety hazards and other considerations during routine operations of maintenance and normal use in a chemical environment.
Non-intrusive mixing devices have been developed to overcome some of the problems associated with intrusive mixing devices. As used herein, the term "non-intrusive" mixing device refers to an object placed in-line to the flow path of a fluid or solution stream to enhance agitation by direct interference. Such devices include, but are not limited to, sparger systems, baffles, fins, in-line spiral pipings, where solution is rotated around a central twisted, or spiral, elongated pipe and/or alteration of a fluid or solution path into a fluid or solution body.
One of the most widely used non-intrusive mixing systems of the prior art involves the use of a sparger head system, as shown in FIG. 1. In this system, a flow line 2 having an end 4 is introduced into a vessel 6 containing a solution body 8 having a solution level 7. Flow line 2 extends either from the top of the holding vessel, as shown in FIG. 1, or may enter through the bottom of the holding vessel. A sparger head 9 is placed at the end of flow line 2 located below the solution level at a specified orientation in the holding vessel. Solution enters the flow line and is delivered through flow line 2 and exits the sparger head into the solution body. A conical dispersion pattern is created in the solution body as illustrated in FIG. 1.
Testing regarding this system has shown that over time as the system approaches a steady state, the flow path of the solution stream entering the solution body is limited as to the volume of material with which it interacts, and separation of solids from the mixed solutions, such as slurry, occurs in regions of little or no agitation. Because the flow pattern of the solution disbursed from the sparger head is conical, and is situated in a limited conical region in front of the sparger, "dead zones" of minimal agitation of the suspension develop over time. These "dead zones" essentially are pockets outside the conical dispersion region where the solution body is not in direct contact with the delivered fluid stream of solution in the holding vessel. Thus, the solution tends to separate into constituent components and become nonhomogeneous in these regions and loses effectiveness. Additionally, regions outside of the conical region, where inadequate mixing occurs, show evidence of build up or caking along the interior peripheral walls of the holding vessel near the base.